Ignitor or Flame Sensor: How to Tell Which One Failed

Two of the most common no-heat calls in the Twin Ports produce completely different sounds at the furnace, and once you know what to listen for you can tell a homeowner's story apart in about thirty seconds. One furnace never lights. The other lights beautifully and then gives up. Knowing which one you have is worth something before you book furnace repair — it tells you whether you are looking at a part that has stopped working or a part that has stopped being believed.

What happens in the 60 seconds after the thermostat calls for heat?

A modern gas furnace runs a fixed ignition sequence, and every step has to prove itself before the next one is allowed to start. That sequence is the diagnostic tool, because a failure always stops it at the same place.

1. The thermostat closes the heat circuit and the control board wakes up.
2. The inducer (draft) motor spins up and pulls air through the heat exchanger and vent.
3. That airflow creates a negative pressure that closes the pressure switch, proving the vent is clear.
4. The board energizes the hot surface ignitor, which glows orange for roughly 15 to 45 seconds.
5. The gas valve opens and the burners light off the glowing ignitor.
6. The flame sensor, a thin metal rod sitting in the flame, passes a tiny current to ground and proves the flame to the board.
7. With flame proven, the board keeps the gas valve open and starts the blower after a delay.

Stop the sequence at step 4 or 5 and you have an ignition problem. Stop it at step 6 and you have a flame sensing problem. Everything else is detail.

What a failed hot surface ignitor looks and sounds like

A failed hot surface ignitor produces a furnace that never lights: the inducer runs, you hear the pressure switch click, there is a pause, and then the inducer winds down without any burner noise at all. Nothing glows inside the burner compartment, and there is no smell of combustion because no gas was ever released.

Hot surface ignitors are silicon nitride or silicon carbide elements, and they fail the way filaments fail — from thermal cycling, not from dirt. In a 9,000–10,000 heating-degree-day climate a furnace cycles far more times per winter than the same unit in a milder place, so ignitor life here is measured in cycles the manufacturer never expected. They also crack from being touched, which is one of several reasons the burner compartment is not a place to poke around.

The other things that produce the same no-light symptom are a control board no longer powering the ignitor, a broken ignitor lead, or a gas valve that is not opening. From outside the furnace all four look identical, which is the honest limit of listening at the basement door.

What a flame sensing problem looks and sounds like

A flame sensing problem produces a furnace that lights and then quits within a few seconds: burners ignite with a soft whoomp, run for roughly two to seven seconds, and then go silent while the inducer keeps running. Usually the furnace tries again. Often it tries three times and then locks out, leaving the blower running to clear heat out of the exchanger.

The flame sensor does not switch anything. It relies on flame rectification: the flame itself conducts a very small direct current from the rod to the burner ground, and the control board measures microamps. A film of silica and oxide on the rod — which builds up gradually over a heating season — drops that current below the board's threshold. The board reads no flame, and it does exactly what it should with an open gas valve and no proven flame: it closes the valve.

That is why the failure feels so dramatic and is usually so small. Nothing is broken. The furnace has lost confidence in a signal and refuses to leave gas flowing on a maybe. The same symptom also comes from a cracked ceramic insulator, a poor burner ground, or a flame drifting off the rod because the burners are dirty, so the sensor is the first suspect and not the only one.

Sorting a no-heat call by what you can hear from the basement stairs

What you observe	Where the sequence stopped	What it usually means
Inducer runs, then everything winds down, no burner noise	Ignition, step 4–5	Ignitor, ignitor circuit, or gas valve
Burners light, quit in a few seconds, furnace retries	Flame proving, step 6	Flame sensing, burner ground, or dirty burners
Inducer runs and keeps running, never any glow	Pressure switch, step 3	Blocked vent or intake, cracked hose, weak inducer
Three tries, then silence and a flashing light	Lockout after repeated failure	The board did its job; the underlying fault is still there
Burners light and stay lit, but the air is cool	Blower or airflow side	A different problem entirely, often airflow or a control fault

If the last row is where you landed, the causes are covered in more detail in our walkthrough of why a furnace blows cold air, which starts from the blower rather than the burners.

Why the control board's flash code is a starting point, not an answer

The flash code on a furnace control board reports what the board observed, not what failed. A board flashing a flame-sense fault is telling you it never saw adequate microamps. That is true whether the rod is fouled, the ground strap is corroded, the burners are carboned up, or the gas pressure is low enough that the flame is lifting off the rod.

A technician reads the code, then verifies it independently: microamp reading at the sensor lead, manifold gas pressure, burner flame pattern, ignitor resistance. In the rural townships this matters more, because a propane furnace with the wrong orifices or a regulator out of adjustment imitates a sensor fault exactly. Houses around Meadowlands and Kerrick are usually on LP rather than natural gas, and fuel-side problems there get misread as parts failures.

What these repairs cost, and what changes the number

Both repairs sit in the lower half of our furnace repair range of $180–$900. That includes the $89 diagnostic, credited toward the work once you approve it. Ignitors and flame sensors are inexpensive parts; what moves the total is access and what else the technician finds.

• A cleaned or replaced flame sensor is the cheapest outcome, and it is a maintenance item rather than a breakdown.
• A cracked ignitor is a straightforward part swap once the burner compartment is open.
• A burner assembly that needs pulling and cleaning adds labor, and is common on furnaces that have gone several seasons without service.
• A control board, gas valve, or inducer motor moves the repair toward the top of the range.
• A furnace wedged into a tight hillside basement in an older house takes longer to work on than one in an open utility room.

Recurring flame sensor calls are worth taking seriously as a signal. A rod that fouls every few weeks is usually telling you about combustion quality or burner condition, and cleaning it again just resets the clock. A fall furnace tune-up that includes combustion analysis is where that gets caught.

What a homeowner can check before calling

There is a short list of things worth checking on a no-heat call, and it stops well before the burners. Work through it, then describe what you heard when you call — the sequence detail tells a dispatcher more than a symptom does.

• Thermostat: set to heat, set point above room temperature, fresh batteries if it takes them.
• The furnace switch on the wall or joist nearby, and the furnace breaker.
• The filter, which when badly loaded can cause a limit trip that looks like something else.
• Outside, the PVC vent and intake terminations on a condensing furnace, which ice over in freezing drizzle here. Clear snow from around them if you can reach them safely from the ground.
• For LP customers, the tank gauge. Houses around Iron River and the Bayfield County townships run out more often than anyone expects during a long cold stretch.

Past that point the diagnosis needs a meter and an open furnace. If the house is cold and dropping, say so when you call — no-heat calls are answered 24 hours a day, and furnace repair in this climate is not something to sit on overnight in January.